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Bureau of Mines Information Circular/1985 




Phosphate Resource Potential for Borehole 
Mining in the Southeastern Coastal Plain 

By George H. Popper, Douglas J. Godesky, and James J. Giambra 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75 



^/NES 75TH Atv^ 



ma 



Information Circular 9043 

Phosphate Resource Potential for Borehole 
Mining in the Southeastern Coastal Plain 

By George H. Popper, Douglas J. Godesky, and James J. Giambra 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Morton, Director 



■M-f0't3 



Library of Congiess Cataloging in Publication Data: 



Popper, George H 

Phosphate resource potential for borehole mining in the southeast- 
ern coastal plain. 

(Information circular ; 9043) 

Bibliography: p. 14-15. 

Supt. of Docs, no.: I 28.27: 9043. 

1. Phosphates— Southeastern States. 2. Boring. 3. Coasts— South- 
eastern States. I. Godesky, Douglas J. II. Giambra, James J. III. 
Title. IV. Series: Information circular (United States. Bureau of 
Mines) ; 9043. 



'm2t5T4J4.^, [TN913] 622s [553.6'4] 84-600385 



;ONTENTS 



Abstract • 

Introduction 

Acknowledgments • 

Borehole mining < 

Borehole mining system 

Deposits suitable for borehole mining , 

Environmental considerations , 

Geology 

Geologic history and phosphate genesis 

Lithology 

Resource data 

Areal extent 

Regional variability 

Overburden , 

Ore zone thickness , 

Grade 

Reserves-resources 

Evaluation method 

Resource estimate 

Potential phosphate rock, product , 

Conclusions 

References 

Bibliography 

Appendix A. — Mining and benef iciation parameters 

Appendix B. — General resource data 

ILLUSTRATIONS 

1. Study area, Southeastern Coastal Plain , 

2. Borehole mining system , 

3. Ancient environments of phosphate deposition , 

4. Depositional basins of Southeastern Coastal Plain , 

5. Isopach map, Hawthorn Formation in Florida , 

6. Isopach map, Pungo River Formation in North Carolina..., 

7. Classification system of mineral reserves and resources, 

TABLES 

1. Borehole mining phosphate resource potential , 

B-1. Resource characteristics of identified deposit zones..., 
B-2. Extent of environmentally sensitive areas , 



Page 



1 


2 


3 


3 


3 


4 


5 


6 


6 


6 


9 


9 


9 


10 


10 


10 


11 


11 


12 


13 


14 


14 


15 


19 


20 



2 

4 

7 

8 

12 

12 

13 



13 

20 
20 



- 


UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


g/cm^ 


gram per cubic centimeter mi 


mile 


ha 


hectare mt 


metric ton 


km 


kilometer mtph 


metric ton per hour 


m 


meter pet 


percent 



I 



PHOSPHATE RESOURCE POTENTIAL FOR BOREHOLE MINING 
IN THE SOUTHEASTERN COASTAL PLAIN 

By George H. Popper, Douglas J. Godesky, and James J. Giambra 



ABSTRACT 

The Bureau of Mines has evaluated the extent of phosphate resources 
available for recovery by the experimental borehole mining method in 
the Southeastern Coastal Plain of the United States. Phosphate re- 
sources at overburden depths greater than 30 m are as a rule currently 
unsuitable for recovery through conventional mining methods because of 
economic, environmental, or technical considerations. In the identi- 
fied deposit areas, borehole mining operations are projected to yield a 
more favorable rate of return and to be environmentally more desirable 
than conventional surface mining. Borehole mining resource estimates 
presented in this study are preliminary and designed to serve as a ba- 
sis for future resource evaluations, as the current data base is rela- 
tively sparse and incomplete. The resulting resources have, therefore, 
been classified as hypothetical and speculative and are subject to up- 
dating as additional exploratory data become available. 

Hypothetical and speculative resources amenable to borehole raining 
are estimated to total about 385 billion metric tons (mt) of phosphate 
matrix with a minimum in situ grade of 5 pet P2^5' With projected 
borehole mining capabilities and current conventional benef iciation 
procedures, this resource would make available approximately 64 billion 
mt of phosphate rock product at an estimated grade of 30 pet P20 5* 

^Physical scientist, former Eastern Field Operations Center, Bureau of Mines, 
Pittsburgh, PA (now with Office of Surface Mining, Reclamation and Enforcement, 
Pittsburgh, PA) . 

^Geologist (now consulting geologist, Lewiston, ME). 



INTRODUCTION 



This Bureau of Mines study is a prelim- 
inary evaluation of the resource poten- 
tial of deep bedded, on-shore phosphate 
deposits projected to be suitable for 
borehole mining. Most of these resources 
are not commercially recoverable under 
current mining and economic conditions. 
However, as a result of diminution of 
easily mined, near-surface resources, as 
well as an increase in environmental con- 
straints, a need was recognized to iden- 
tify and evaluate deep-bedded phosphate 
deposits; these deposits may be suitable 
for recovery in the future using a com- 
mercial borehole mining system. 

The area of study comprises the South- 
eastern Coastal Plain of the United 
States as illustrated in figure 1. The 
study area includes the on-shore deposits 
of the Atlantic and Gulf Coastal Plains. 
Because of the exceptionally large size 
of the study area, limited amount of data 



/ iJomes 

) Petersburg ^^^^^..^^^f^n^ 

/Virginia J^S^^^^-J^ ■ ^^^r 
/_____ .i--^"^ f?>^^ ftf^ AlbermoMe Sound 

^^Cofie Hatteros 



/ Raleigh uy^ — 

/^\ 'fiver 



y North ^ / I'e^-^^'—^Xv,/ 

.y^ Carolina v ^ / <:, '^ ^'•''"Z^ 

( ^ 'vC^ ^i/-"^ Cape Lookout 



-■r- ' South Va^"? 
C,_ Carolina <<V \\ 
i j \ Columbia__t^ ^ 

\ Georgia ^-^ /' '^ JiU'e, 

\ / Augusla^i! ^ ^\ief 

V ^ t^^H". •/- 

M ,. y XO ^\% ^Charleston 

i Macon f , o~^ M% "^ 

Columbus (^\^ama^(, • Savannah 

■ ■ ' •. Jacksonville 

Tallahassee ■ 



Wilmington 
Cope Fear 




Atlantic Ocean 



• Palm Beach 



• Miami 60 120 mi 
I h h 



100 200 km 
Scale 



FIGURE 1. - Study Area, Southeastern Coastal Plain. 



available on deep-bedded phosphates, and 
the synthesis of current authoritative 
interpretations of the data, the result- 
ing tonnage estimates have been catego- 
rized as hypothetical and speculative re- 
sources. The distribution and estimated 
tonnages of phosphate resources should 
therefore be used as a basis for further 
refined exploration and evaluation of 
borehole minable deposits rather than as 
a conclusive and final evaluation. 

Previous investigations by the Bureau 
have indicated that the experimental 
borehole mining system should prove to be 
an economically and environmentally at- 
tractive method of recovering commercial 
quantities of phosphate under certain 
conditions (9).^ The principal condition 
being that the phosphate deposit occurs 
at a depth too great to be considered 
economically recoverable by conventional 
mining and existing technology. An addi- 
tional attribute of borehole mining is 
that it can be employed in areas where 
conventional surface mining would be aes- 
thetically or otherwise objectionable. 

The feasibility of the borehole mining 
system for recovering phosphate ore (ma- 
trix) was field tested under a Bureau 
contract (16). In the initial tests, 
about 1,500 mt of matrix was recovered at 
an overburden depth of 75 m. A prototype 
borehole mining system, designed as part 
of the contract, is the basis for the 
borehole mining parameters used in the 
present study. Although the prototype 
system has not yet been built or tested, 
the use of these design specifications 
and parameters provides a preliminary 
method for projecting actual mining re- 
sults and resource recovery. Any inter- 
pretation of mining and resource data 
from this study should be utilized in 
this context. 

Test results of a conventional benefi- 
ciation process to concentrate phosphate 
matrix mined by borehole methods were 
used to estimate the tonnage of potential 

^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the bibliography. 



phosphate product. Matrix is the natural 
occurrence of unbenef iciated phosphate 
ore. It consists of the calcium phos- 
phate mineral apatite with quartz, cal- 
cite, and dolomite; clay and iron oxide 
minerals comprise the gangue within the 
matrix (_5) • Washing and double-stage 
flotation are proposed to produce a mar- 
ketable phosphate rock product. 

The phosphate resource estimate pre- 
sented in this report is based upon an 
analysis and extrapolation of public, in- 
dividual, and industry data. The data 
base is relatively sparse and its quality 
variable. An extensive bibliography is 
included to serve as a source of current 
phosphate resource information for the 
Southeastern Coastal Plain and reflects 



much of the background data and research 
used in this study. 

The location and configuration of an- 
cient phosphate depositional environ- 



ments were considered 
factors controlling 
of existing deposits, 
were delineated and 
thicknesses, grades, 
signed to each area. 



to be the primary 

the distribution 

Paleoenvironments 

average deposit 

and densities as- 

Site-specif ic drill 



hole data were utilized but are not pre- 
sented here because their inclusion would 
not be correlatable with the area deter- 
minations as often the drilling record 
was incomplete and extrapolation was nec- 
essary. Resource calculations and esti- 
mates were then made for three major 
areas of phosphate deposition. 



ACKNOWLEDGMENTS 



Grateful acknowledgement is made to the 
following individuals for their contribu- 
tions of data and access to information, 
which made this study possible: J. P. 
Bernardi, exploration manager. Interna- 
tional Minerals and Chemical Corp. , Bar- 
tow, FL; R. C. Fountain, geological con- 
sultant. Winter Haven, FL; H. Gill and 
M. Hacke, U.S. Geological Survey, Dora- 
ville, GA; R. B. Hall, Gardinier Corp., 
Fort Meade, FL; V. J. Henry, chairman. 



Geology Department, Georgia State Univer- 
sity; Atlanta, GA; P. Huddleston, geolo- 
gist, Georgia Geological Survey, Atlanta, 
GA; J. A. Miller, U.S. Geological Sur- 
vey, Atlanta, GA; S. R. Riggs, geologist. 
Department of Geology, East Carolina 
University, Greenville, NC; and T. M. 
Scott, geologist, Florida Bureau of Geol- 
ogy, Tallahassee, FL. The final opinions 
and conclusibns expressed here, however, 
only represent those of the authors. 



BOREHOLE MINING 



BOREHOLE MINING SYSTEM 

Borehole mining is an experimental 
method of mining that employs highly 
pressurized water to form an ore slurry 
at depth. To date, field tests have been 
successful and additional industry pilot- 
scale work is planned. Through use of 
this technique, phosphate matrix can 
be recovered from deep horizons with- 
out requiring the removal of overbur- 
den. The experimental borehole mining 
system, as illustrated in figure 2, may 
be carried out under either open- or 
f looded-cavity conditions. In the pro- 
posed borehole raining system, a borehole 
mining tool is lowered to the phos- 
phate horizon through a predrilled steel- 
cased borehole. A rotating water jet on 



the tool disaggregates the phosphate ma- 
trix while a jet pump at the lower end of 
the tool pumps the resulting slurry to 
the surface. The slurry is then trans- 
ported to a benef iciation plant by pipe- 
line. The resulting cavity is backfilled 
with waste material (tailings) from the 
plant. 

Application of the borehole raining sys- 
tem is not limited by matrix depth (9). 
Initial tests indicate that although 
slurry recovery rates vary, 4 5 mtph of 
matrix is a reasonable estimate of a com- 
mercially obtainable mining rate. For 
this evaluation, overall mining recovery 
is estimated to be 66 pet. These figures 
may vary depending upon site-specific 
conditions, modifications in mining tech- 
niques, or results of future tests. 



Beneficiation 
plant 



Tailings return 
to backfill 




Backfilled 
material 
(tailings) 



FIGURE 2. - Borehole mining system. {Courtesy Flow Industries, Inc.) 



Drilling and mining would be conducted 
on a staggered grid pattern. The re- 
sulting cavities are expected to be 
bowl-shaped and about 18 m in diameter. 
A spacing of approximately 21 m between 
boreholes would leave a final barrier of 
3 m between mined-out cavities. Antici- 
pated borehole mining unit specifications 
and operating parameters are summarized 
in appendix A. 

DEPOSITS SUITABLE FOR BOREHOLE MINING 

Initial tests conducted for the Bu- 
reau indicated that relatively competent 
strata must immediately overlie phosphate 
matrix in order to carry out successful 
borehole mining under open-cavity condi- 
tions. This was thought necessary in 
order to avoid cavity collapse. Results 



of subsequent tests suggested that it may 
be possible to mine deposits where only 
minimal lithologic differences exist 
between the ore zone and the overburden 
provided that the operation is carried 
out under f looded-cavity conditions. 
Some contrast in lithology must exist be- 
tween the matrix and the overburden in 
order to afford a bearing surface for the 
water in the cavity. 

Cavity roof conditions in the resource 
areas identified were not determined as 
part of this study. Future exploration 
and tests may determine that some areas 
must be deleted from the identified 
resource areas if overburden conditions 
are such that borehole mining would not 
be successful. Site-specific tests will 
have to be made to determine each depos- 
it's suitability for borehole mining. 



Primary phosphate deposits, deposits 
in which no appreciable reworking or 
leaching have taken place, are more suit- 
able for mining by the borehole method 
than those in which secondary reworking 
has taken place. Primary deposits tend 
to be consistent in thickness and uniform 
in the quality and distribution of the 
matrix; these factors facilitate explora- 
tion and make mining easier to plan. 

When mining at depths shallower than 
30 ra, conventional mining methods are es- 
timated to yield higher rates of return; 
at depths of 45 m and greater, it is an- 
ticipated that borehole mining would be 
the more economical method (_9 ) . In addi- 
tion, the borehole technique is environ- 
mentally less disruptive at all depths. 
The rate-of-return threshold between con- 
ventional and borehole raining, where 
borehole mining is projected to yield a 
higher rate of return, is estimated to 
occur between 30 and 4 5m. The present 
study evaluates the resource potential of 
phosphate deposits at overburden depths 
of at least 30 m. A precise deposit 
depth at which conventional mining sys- 
tems become economically inferior or 
technically unfeasible has not been de- 
termined for the deposits of this large 
study area. A determination of that na- 
ture would vary between mining zones and 
conditions within the coastal plain, but 
will most likely occur between 30 and 
4 5 m. 

ENVIRONMENTAL CONSIDERATIONS 

The borehole mining system would elimi- 
nate the need for the removal of overbur- 
den. Site cleanup, revegetation, and 
cavity backfilling are the only reclama- 
tion procedures required. 

In using the borehole mining system, 
benef iciation waste is backfilled into 
mined-out cavities and thus long-term 
aboveground waste storage is avoided. 
Water used in borehole raining and benefi- 
ciation is recirculated within the sys- 
tem. Initial tests suggest also that 
ground water drawdown and surface subsi- 
dence would be minimal or insignificant 



(16). Applications of borehole raining in 
areas of relatively shallow overburden 
may have raore profound environmental 
effects. 

Borehole mining in the Southeastern 
Coastal Plain would be directed toward 
recovering phosphate from the Miocene 
Hawthorn and Pungo River Formations. The 
Hawthorn is underlain in many areas by a 
thick section of carbonate rocks that 
constitute the principal aquifer. Mining 
of the basal Hawthorn Formation would 
therefore have to be done along with mon- 
itoring of any underlying aquifer (19). 

A major regional source of ground wa- 
ter for the study area is the Coastal 
Plain aquifer, formed by limestones that 
dip gently seaward and are covered by 
imperraeable Miocene clays (1). Ground 
water is not normally free to move 
vertically through an aquiclude that con- 
fines a ground water system. A clay 
aquiclude 7 to 11m thick underlies the 
Upper Miocene phosphate matrix found 
southeast of Savannah, GA; the clay is 
believed to be capable of locally pro- 
tecting that aquifer from deleterious 
effects of mining in that region (1). 
Such favorable determinations would be 
required in other deposit areas prior to 
the initiation of any commercial mining 
operations. 

Borehole mining may not be possible or 
desirable in areas such as barrier is- 
lands, tidal marshes, flood plains, and 
wildlife refuges, where environmental 
damage can be severe or industrial usage 
undesirable. In addition, urban areas, 
military bases, major water areas, and 
State and Federal forests are not consid- 
ered likely mining zones because of land- 
use conflicts. In order to get a more 
reliable resource estimation, such areas 
were deleted from the resource calcula- 
tions wherever possible. Approximately 
1.9 million ha was deleted from the ini- 
tial resource area. An estimate of the 
areas susceptible to environmental dam- 
age or land-use controversy within the 
identified deposit zones is given in 
appendix B. 



GEOLOGY 



GEOLOGIC HISTORY AND PHOSPHATE GENESIS 

The Southeastern Coastal Plain of the 
United States was the site of widespread 
phosphate deposition during the Miocene. 
Phosphate was deposited discontinuously 
from Virginia to southern Florida on a 
shallow marine shelf of low relief. Re- 
sulting phosphate deposits occur in the 
Miocene Hawthorn, Pungo River, and corre- 
lative formations. Subsequent to deposi- 
tion, there was little modification of 
the Miocene sequence other than minor 
ground water and sink hole development; 
structural deformation was minimal (14) . 

The deposition and distribution of 
phosphate along the Atlantic Coastal 
Plain was controlled by the availabil- 
ity of phosphate from deep ocean cur- 
rents, glacially and tectonically pro- 
duced fluctuations in sea level, and the 
paleogeomorphic setting ( 13 , 1 5) . Sig- 
nificant precipitation and accumulation 
of sedimentary phosphorites took place 
only under appropriate combinations of 
this complex set of tectonic and environ- 
mental variables ( 14 ) . 

Deposition was locally cyclic in na- 
ture and attributable to the following 
sequence of events. As a result of nor- 
mal marine circulation, cold waters, en- 
riched with phosphate from both organic 
and inorganic sources, upwelled against 
the eastern coastal margins. Longshore 
currents transported the cold phosphate- 
enriched waters southward along the 
ancient coastline. Material deposited 
during this time consisted primarily of 
terrigeneous matter with subordinate 
amounts of phosphate; resultant phosphate 
concentrations are thin and of low grade. 
As glaciers melted and the sea level 
rose, cold, deep waters transgressed lo- 
cal portions of the Atlantic coastal 
shelf. Where these currents encoun- 
tered topographically raised areas, sig- 
nificant concentrations of phosphate 
were deposited in the warmer shallower 
waters. Phosphate accumulated in coastal 
basins adjacent to the ancient topo- 
graphic highs. Units resulting from 
these conditions consist predominantly 



of phosphate with terrigeneous material 
subordinate. 

As sea level continued to rise, the 
axis of the ancient Gulf Stream veered 
westward. Portions of the shelf were 
thus flooded with warm Gulf Stream wa- 
ters. As the cold, upwelling, phosphate- 
producing environments were destroyed, a 
discontinuous carbonate caprock was de- 
posited instead. With the onset and de- 
velopment of a new glacial period or tec- 
tonic event, sea level fell and erosion 
proceeded to remove portions or, in some 
cases, even the entire thickness of the 
previously deposited units (13). 

The shape of the phosphate deposits in 
the Southeastern Coastal Plain is direct- 
ly related to the geomorphology at the 
time of deposition. The troughs in which 
the phosphate was deposited characteris- 
tically were linear depressions or en- 
trapment basins adjacent to associated 
topographic highs (14). Phosphate depos- 
its in these zones are thin along the 
flanks of the arches and thicker toward 
the axis of the trough. The ancient top- 
ographic highs tend to be oriented either 
northwest or northeast and are present 
throughout the length of the phosphate 
province. The phosphate-forming environ- 
ments of the Southeastern Coastal Plain 
are illustrated in figure 3. The struc- 
tural controls are also shown. These 
ancient features all influenced the pat- 
terns of phosphate distribution by con- 
trolling the formation and regional accu- 
mulation of Miocene phosphorite. 

LITHOLOGY 

Phosphate occurs in the Southeastern 
Coastal Plain as phosphorite, a sedimen- 
tary rock containing at least 5 pet P2O5 
(14). Phosphorites are composed in part 
of phosphate particles that may range 
from pebble to clay size. Generally, 
phosphatic particles are associated with 
quartz grains, carbonate grains, or clay 
minerals. The phosphorite may be in- 
terbedded with quartz sands, dolomite, 
magnesium-rich clays, or diatomite. 



^^v^ 



Mioce ne U pland 





LEGEND 

Areas suitable for borehole mining 
Hawthorn- Pungo River 
Formations - deep environment 



Hawthorn -Pungo River 
Formations- intermediate 
environment 

Howthorn -Pungo River 
Formations- shallow 
environment 

Areas unsuitable for borehole mining 
Phosphatic horizons 
J absent, shallow, or discontinuous 



Miocene Upland 

,--' Limit of borehole minability 

ij_ Structure contours on the base 
of the Hawthorn Formation, 
depth in meters 



40 80 

1 ' ' 'i ' ' ^^-^-^ 
100 

Scale 



120 

-1 i_ ^ 



mi 



200 km 



FIGURE 3. 
14 and 11;. 



Ancient environments of phosphate deposition (modified from references 



1 



Aurora 
Embayment 



LEGEND 

- Form lines, define the 
shape of the ancient 
basins. 




1 



200 km 



South Florida 
Embayment 



FIGURE 4. 
14 and 11). 



Depositionol basins of Southeastern Coastal Plain (modified from references 



RESOURCE DATA 



AREAL EXTENT 

Phosphate occurs in Miocene strata 
throughout the entire Southeastern Coast- 
al Plain. Those strata having the great- 
est resource potential for borehole 
mining occur from northeastern North Car- 
olina to southern Florida; they are 
termed the Pungo River Formation in North 
Carolina and the Hawthorn Formation in 
South Carolina, Georgia, and Florida. 

The delineation of phosphate deposits 
of sufficient concentration, occurring at 
depths of at least 30 m, has resulted in 
the identification of three major deposi- 
tional regions. These are illustrated in 
figure 4. The primary regions for bore- 
hole minable deposits are the Aurora Em- 
bayment of North Carolina, a region lo- 
cated eastward and downdip of the current 
North Carolina phosphate fields; the 
Southeast Georgia Embayment in Georgia 
plus the Jacksonville Basin and St. Johns 
Platform of northeastern Florida; and the 
Central Florida Platform, Brevard Plat- 
form, Osceola Basin, and Okeechobee Basin 
of southeastern and central Florida. In 
this report, these areas are respectively 
designated as eastern North Carolina, 
Southeast Georgia Embayment, and southern 
Florida. 

An area of about 7.1 million ha is un- 
derlain by matrix considered suitable for 
borehole mining. The extent of favorable 
matrix within each area is presented in 
appendix B. Specific exploratory data 
that are the basis for the delineation of 
the deposits and region boundaries range 
from locally dense and reliable to sparse 
and interpretive. Specific drilling, 
mapping, geologic, and other explora- 
tory data cannot be presented in a quan- 
titative way in order to duplicate the 
resource conclusions without the qualify- 
ing interpretations by authorities on the 
particular data. 

Within each depositional region, indi- 
vidual phosphate beds may not be contin- 
uous over the entire area, yet suitable 
matrix is assumed to exist throughout 
each depositional environment. Specific 
locations within a region may exhibit 
multiple beds of matrix, whereas others 



may contain only a single bed or no 
matrix. The estimated average matrix 
thicknesses presented are representative 
of total combined beds, and published re- 
gional interpretations and summary data. 
The matrix thickness should not be misin- 
terpreted to indicate blanket deposition 
or deposits. 

The potential for suitable phosphate 
deposits north of Alberraarle Sound, NC, 
was discounted because of the low phos- 
phate and high glauconite content that 
characterize the units of that area. 
Miocene strata of southwestern Georgia 
are probably not suitable as a potential 
resource because of their low phosphate 
content. Hard-rock phosphate deposits 
overlying the Ocala Arch' in Florida are 
too shallow and discontinuous. 

No determination was made for the 
region south of Lake Okeechobee, FL, 
because reliable data on phosphate 
bearing horizons in this area are not 
available. Deep-bedded phosphate is al- 
so reported to occur in small deposits 
outside of the identified zones, but 
data are sparse and the phosphorite is 
thought to be of limited lateral extent 
(Z). 

REGIONAL VARIABILITY 

Phosphate-bearing strata exhibit a re- 
gional variability along the length of 
the Southeastern Coastal Plain. The Mio- 
cene section tends to thicken and deepen 
eastward in North Carolina, Georgia, and 
northern Florida; in southern Florida 
the section becomes thicker and deeper 
southward. 

Generally, the percentage of carbonate 
units increases southward toward southern 
Florida where they represent the dominant 
lithology. By contrast, the clastic 
material content, mostly terrigeneous 
sands, increases northward and consti- 
tutes the dominant lithology in North 
Carolina. Carbonates, however, are also 
present in North Carolina where they 
occur principally as caprock horizons 
within the terrigeneous sequence. 

Diatom content increases northward. 
Glauconite appears in the Pungo River 



10 



Formation and increases northward until 
it becomes a major component of strata 
north of Albemarle Sound, NC. 

Near the ancient topographic highs or 
ancient shore lines, the phosphorite is 
characterized by erratic areal distri- 
bution and a high P2O5 content. In the 
deeper depositional environments, al- 
though the phosphorite is thicker, the 
P2O5 content of the pellets is lower, the 
magnesium content is greater, and the 
average grain size is smaller. The phos- 
phorites in downdip areas have not been 
modified by supergene weathering and 
consequently have not been chemically 
upgraded (14). Dolomite content tends 
to increase toward the deeper portions 
of the depositional basins; the equiva- 
lent facies in shallow water consist of 
magnesium-rich clays. 

Phosphate differs in character across 
the Southeast Georgia Embayment. Depos- 
its of the Savannah area are similar to 
those of North Carolina but distinctly 
different from concentrations along the 
west and south margins of the embayment 
in south Georgia (6). Phosphate of the 
Savannah area is characterized by dark 
color, low pebble content, and high iron 
and aluminum content. 

In Florida, the southern extension of 
the Central District contains vast re- 
serves of relatively low P2O5 grade mate- 
rial, contained predominantly within an 
upper clastic section of the Hawthorn 
Formation (8). In addition, the MgO con- 
tent is relatively high. In this area 
the clastic phosphorite overall grain 
size decreases southward as the distance 
from the source area increases. As with 
most of the resource areas, the phospho- 
rite beds of the Hawthorn in southern 
Florida vary significantly in thickness 
and distribution, and change facies rap- 
idly both vertically and laterally, yet 
the total upper clastic section has suf- 
ficient vertical and horizontal continu- 
ity to be considered a single unit (2). 

OVERBURDEN 



Formation and related matrix averages 
over 30 m deep (4_) . No attempt was made 
to determine the viability of the over- 
burden for cavity support as this re- 
quires site-specific evaluation. Initial 
testing of the borehole mining system 
indicated that mining must be conducted 
in a flooded cavity in order to avoid 
caving. Some contrast in lithology must 
exist between the ore zone and over- 
burden in order to afford a bearing sur- 
face for the pressurized water in the 
cavity. 

ORE ZONE THICKNESS 

Borehole mining may be applied to 
ore zones as thin as 0.33 m; in this 
study, all identified matrix was at least 
0.33 m thick. Phosphorite zones gener- 
ally range in thickness from about 0.33 
to 6 m; in some instances they are 15 m 
thick, rarely do they exceed 30 m (14). 
Representative thicknesses are given in 
appendix B for each depositional environ- 
ment within a basin. 

GRADE 

A P2O5 content of 5 pcf^ was considered 
the minimum grade necessary for a phos- 
phatic bed to be classified as a po- 
tentially minable zone of matrix. An 
estimated average of 10 pet P2O5 was used 
for the calculation of potential product 
tonnages. Matrix currently being mined 
in Florida has a feed grade of 10 to 
15 pet P2O5 (20). 

It is anticipated that the MgO content 
of the matrix recovered in much of the 
resource area will be higher than the 
levels in matrix currently being recov- 
ered in Florida and North Carolina. A 
high-MgO content is primarily attribut- 
able to the presence of dolomite grains 
in the matrix. The inclusion of MgO in 
phosphate flotation concentrate creates 
problems during the production of phos- 
phoric acid and other products. Most of 
the remaining near-surface deposits in 



In all cases , depth to the ore zones 
evaluated in the present study is at 
least 30 m. The maximum overburden depth 
is over 200 m. In Florida, the Hawthorn 



^1 pet P2O5 X 2.184 = 1 pet BPL; BPL is 
bone phosphate of lime, used to express 
the phosphate content of matrix or bene- 
ficiated product. 



11 



central Florida contain higher than de- 
sirable levels of dolomite (J^) . Thus, 
the problems associated with high-MgO 
content are not limited only to borehole 
minable deposits. 

Areas have not been eliminated from 
the resource estimation because of high- 
MgO levels, as benef iciation tests have 
demonstrated that it is possible to re- 
move up to approximately 93 pet of the 
total MgO in matrix flotation feed (10). 
Other methods of reducing the MgO content 
such as blending and high-, low-grade 
zone mining can also be utilized. Site- 
specific benef iciation tests would 
have to be conducted in order to evalu- 
ate the potential amount of MgO in the 
product. 



Generalized P2O5 recoveries for benef i- 
ciation of high-MgO matrix have been used 
to estimate potential product tonnages 
for such areas. Southern Florida has 
been evaluated as a high-MgO resource 
area since carbonates are the most sig- 
nificant and extensive contaminant of the 
Hawthorn phosphorites in that region (2). 

Phosphate pebble is a minor constituent 
of Hawthorn Formation elastics, making up 
less than 7 pet of the total (2^, 8^). De- 
posits of low pebble content are more 
amenable to borehole mining and such rel- 
atively low pebble contents are charac- 
teristic of primary phosphate deposits. 
Alteration by leaching is not extensive 
in the deep bedded Hawthorn Formation and 
equivalent strata. 



RESERVES-RESOURCES 



EVALUATION METHOD 

Calculation of the resource estimate 
was based upon two factors: the limits 
of the environments of phosphate deposi- 
tion and the physical characteristics of 
the matrix in single or multiple beds 
(i.e., areal extent, thickness, grade, 
and density). Environments were deline- 
ated based upon the existing structure 
of the Miocene Basin. The environmental 
depth limits (0, 50, 122 m) are those 
suggested in reference 14 and are based 
on structure contours at the base of the 
phosphorite-bearing unit. The shallow 
environment encompasses the area between 
the 0- and 50-m contours. Since this 
study is solely an investigation of deep- 
bedded phosphates, only the area between 
30 and 50 m was evaluated for the shallow 
environment. The intermediate environ- 
ment extends from 50 to 122 m, and the 
deep environment is represented by units 
below 122 m. These boundaries are only 
an approximation of the Hawthorn bathym- 
etry, for through time the environmental 
limits must have shifted markedly across 
the basin in response to fluctuations 
in sea level. An attempt has been made 
to account for such variations in this 
evaluation. 

The physical characteristics associated 
with each environment were established 
based upon a synthesis of published and 



proprietary information, well data, 
drillers logs of wash borings, core de- 
scriptions, personal communications, and 
projections of regional trends. Exten- 
sive use was made of well logs and 
isopach and structure contour maps. 
Site- and test-specific geologic explora- 
tory data used in this study have not 
been duplicated lest they be misinter- 
preted to indicate that the conclusions 
are entirely quantitative and can be sub- 
stantiated without significant regional 
interpretation. 

Hypothetical and speculative resource 
areas were delineated based upon the den- 
sity and reliability of data and con- 
clusions of individuals most familiar 
with the basins. Known variations in the 
physical characteristics of the phosphate 
zones within a specific environment were 
used to project the matrix characteris- 
tics into deposit areas with limited 
data. Average deposit thicknesses were 
developed for each depositional environ- 
ment and then applied to the hypotheti- 
cal and speculative resource areas. The 
final resource estimation is based upon 
the equation 

r = a X 10,000 x t x d, 

where r = in situ resource, mt , 

a = resource area, ha. 



12 



and 



t = matrix thickness, m, 
d = density, g/cm-^ 



A density of 1.44 g/cm^ was used to 
approximate the overall regional matrix 
density. The regional data used in the 
resource evaluations are presented in ap- 
pendix B. As stated previously, within 
each depositional region, single or mul- 
tiple phosphate beds may not be continu- 
ous over the entire area, yet sufficient 
vertical and horizontal matrix continuity 
is assumed to exist throughout each depo- 
sitional environment for resource calcu- 
lation requirements and recognition as a 
minable unit. 

The regional variation in thickness of 
the Hawthorn Formation in North Carolina 
and Florida is presented in isopach maps 
(figs. 5-6). Isopach maps have been de- 
veloped and presented with some variation 




Contour interval = 50 m 



.^■ 



FIGURE 5. - Isopach map, Hawthorn Formation 
in Florida (modified from and courtesy of Florida 
Bureau of Geology, 1983). 



by others (_5) . This variation can be 
attributed to differences in the quality 
and density of control data and varia- 
tions in the interpretation of the Mio- 
cene section. 

RESOURCE ESTIMATE 

The principles of the sedimentary phos- 
phate resource classification system of 
the Bureau and the U.S. Geological Survey 
(18) were used in this study. According 
to this system, the resources identified 
here are classified as undiscovered, hy- 
pothetical or speculative. The classifi- 
cation system is presented in figure 7. 

Hypothetical phosphate resources repre- 
sent extensions of known phosphate bodies 
that may reasonably be expected to exist 
in the same region under analogous geo- 
logic conditions. Hypothetical resources 
are based upon drill hole data that have 
been projected for distances greater than 
1 .6 km. 

Speculative phosphate resources occur 
in favorable geologic settings where 
phosphate discoveries have not been made 




10 20 30 40 km 
Scole 



FIGURE 6. - Isopach map, Pungo River Formation 
in North Carolina (modified from reference 12). 



13 



production 



ECONOMC 



MARGINALLY 
ECONOMK 



SUB- 
ECONOMIC 



IDENTIFIED RESOURCES 



Demonstrated 



Measured Indicated 



Inferred 

reserve 

base 



UNDISCOVERED RESOURCES 



Ptobobility range 



Hypothetical 



(of)- 



Speculative 



+ 
+ 



: EC-REE OF GEOLOGIC ASSURANCE 

FIGURE 7, - Classification system of mineral 
reserves and resources. 

and drill hole data are not available. 
If exploration reveals additional data 
about the quality, grade, and quantity, 
these hypothetical and speculative re- 
sources would be reclassified as identi- 
fied resources. 

It is estimated that the Southeastern 
Coastal Plain contains a hypothetical re- 
source of 250.355 billion mt and a specu- 
lative resource of 135.303 billion mt of 
phosphate matrix projected to be suit- 
able for recovery by a commercial bore- 
hole mining system when developed. Re- 
source data for each depositional zone 
are presented in appendix B and are sum- 
marized in table 1. The borehole-mining- 
recoverable resources are estimated to be 
165.234 and 89.300 billion mt of matrix 
for hypothetical and speculative classes, 
respectively . 



POTENTIAL PHOSPHATE ROCK PRODUCT 

The Bureau conducted benef iciation 
studies on phosphate matrix mined by the 
borehole technique (3). Standard double- 
stage flotation, the primary beneficia- 
tion method used by the phosphate indus- 
try, yielded a P2O5 recovery of 91.0 pet 
on ground ore. Standard washing with a 
93-pct-P205 recovery and the 91.0-pct 
flotation recovery are applied here to 
approximate the recoverable phosphate 
rock product in most of the study area. 
For southern Florida, where matrix is an- 
ticipated to have a relatively high MgO 
content, results of other Bureau research 
were used to approximate P2O5 recoveries 
(10). For high-MgO matrix, scrubbing is 
anticipated to yield approximately 76 pet 
P2O5 recovery; flotation, is anticipated 
to yield approximately a 75-pct-P205 
recovery. Employing such benef iciation 
procedures, the borehole mining hypo- 
thetical and speculative recoverable re- 
sources would make available 43.070 bil- 
lion and 21.222 billion mt of phosphate 
rock product, respectively, at an esti- 
mated grade of 30 pet P205« The poten- 
tial phosphate rock product can be ap- 
proximated by the following equation: 



P = t X g, 



where p = product, mt , 



/ g2 



TABLE 1. - Borehole mining phosphate resource potential, 
Southeastern Coastal Plain, million metric tons 



Primary deposit 


In situ matrix 


Recoverable matrix 


30-pct-P205 rock product 


regions 


Hypo- 
thetical 


Specu- 
lative 


Hypo- 
thetical 


Specu- 
lative 


Hypo- 
thetical 


Specu- 
lative 


Eastern North 
Carolina 


123,257 

40,346 
28,687 
58,065 


NAp 

61,776 

8,309 

65,218 


81,350 

26,628 
18,933 
38,323 


NAp 

40,772 

5,484 

43,044 


22,941 

7,509 
5,339 
7,281 


NAp 
1 1 498 


Southeast Georgia 
Embayment 


Northeast Florida'... 
Southern Florida^.... 


1,546 
8,178 


Total 


250,355 


135,303 


165,234 


89,300 


43,070 


21,222 



NAp Not applicable. 



Ix- 



Northeast Florida is combined with Southeast Georgia Embayment to constitute 1 ma- 
jor area of deposition. 

High MgO benef iciation recoveries applied in southern Florida. 



14 



and 



t = in situ matrix, mt, 



gl = matrix grade, pet P2O5, 
g2 = product grade, pet P2O5, 
r = process P2O5 recovery, pet. 



Benef iciation parameters are summarized 
in appendix A. The regional distribution 
for the potential phosphate rock product 
estimate is presented in table 1. 

Another recent estimate of the hypo- 
thetical resources for the study area 
is about 14.27 5 billion mt of recover- 
able phosphate rock (_5) . This figure 
and others found in the literature 



such as an estimate of 25 billion tons, 
hypothetical phosphate resource in the 
entire United States (17) , do not corre- 
spond with the estimate in this study 
because of differences in evaluation cri- 
teria such as maximum depth to the ore 
zones, deposit areas, and mining and 
recovery methods. Also, such resource 
estimates almost exclusively reflect sur- 
face minable resources. At the other end 
of the resource estimate spectrum, poten- 
tial phosphate rock resources for the 
Hawthorn Formation in Florida alone have 
been estimated at 181 to 218 billion mt 
and about 45 billion mt for an area from 
northeast Florida through North Carolina 
(4). 



CONCLUSIONS 



The Southeastern Coastal Plain of the 
United States is estimated to contain 
a hypothetical phosphate resource of 
about 2 50 billion mt and a speculative 
resource of about 135 billion mt of phos- 
phate matrix in approximately 5.2 million 
ha which is projected to be suitable for 
the application of a commercial borehole 
mining system when developed. With esti- 
mated mining and benef iciation recov- 
eries, this total resource would make 



available approximately 64 billion mt of 
30-pct-P2O5 phosphate rock product. This 
enormous on-shore phosphate reserve po- 
tential justifies continuation and expan- 
sion of exploration programs and devel- 
opment of a commercial borehole mining 
system. The application of a commercial 
borehole system would help to assure an 
adequate long-term domestic supply of 
phosphate with minimal effect upon other 
resources and the environment. 



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15 



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Conserv. and Development, Raleigh, NC, 
1965, pp. 430-437. 

Sweeney, J. W. , and R. N. Hasslacher. 
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U.S. Bureau of Mines, Staff. The Flor- 
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BuMines IC 8914, 1983, 42 pp. 

Valentine, P. C. Upper Cretaceous Sub- 
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234 pp. 

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80-3, 1979, 30 pp. 

White, W, A. The Geomorphology of the 
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Woolsey, J. R. , Jr. Neogene Stratig- 
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Coastal Shelf. Ph.D. Dissertation, Univ. 
GA, Athens, GA, 1977, 222 pp. 

Zellars, M. E. The Genesis and Occur- 
rence of Tertiary Phosphorites in the 
Southeastern United States. Min, Eng. , 
V. 30, No. 12, Dec. 1978, pp. 1652-1656. 



Zellars, M. E., and J. M. Williams. 
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Inc.). BuMines OFR 14-79, 1978, 65 pp. 



19 



APPENDIX A. —MINING AND BENEFICIATION PARAMETERS 



BASIC DEPOSIT ASSUMPTIONS 

Matrix grade, pet: 

P2O 5 10. GO 

BPL 21.84 

GENERAL BENEFICIATION PARAMETERS 

MgO content 

Low High 
Recovery, pet: 

Washing-scrubbing 93.0 76.0 

Flotation 91.0 75.0 

Overall benef iciation. . . . 84.6 57.0 
Product grade, pet: 

P2O5 30.00 30.00 

BPL 65.52 65.52 



BOREHOLE MINING UNIT-SPECIFICATIONS 
AND OPERATING PARAMETERS 

Average unit mining rate. . .ratph. . 45 

Average cavity radius m.. 9.1 

Cavity separation m. . 3.0 

Borehole separation m.. 21.3 

Mining recovery pet.. 66 






20 



APPENDIX B. —GENERAL RESOURCE DATA 
TABLE B-1. - Resource characteristics of identified zones 



Region and depositional environment 


Resource 


area, ha 


Av matrix 
thickness , m 


In situ 
10^ 


matrix, 
mt 




Hypo- 
thetical 


Specu- 
lative 


Hypo- 
thetical 


Specu- 
lative 


Hypo- 
thetical 


Specu- 
lative 


Eastern North Carolina: 

Shal low 


97,700 
256,900 
263,900 


NAp 
NAp 
NAp 


6.1 
12.2 
18.3 


NAp 
NAp 
NAp 


8,582 
45,132 
69,543 


NAp 


Intermediate. ...........••••••••••• 


NAp 
NAp 


Deep 


Total 


618,500 


NAp 


NAp 


NAp 


123,257 


NAp 


Southeast Georgia Embayment: ^ 

Shallow 


NAp 
161,200 
297,200 


236,300 
927,500 
266,200 


NAp 
3.0 
7.8 


3.0 
3.0 
3.0 


NAp 

6,964 

33,382 


10,208 


Intermediate. •••••••••••••••••••••• 


40,068 


Deep 


11,500 


Total 


458,400 


1,430,000 


NAp 


NAp 


40,346 


61,776 


Northeast Florida: ' 

Shallow 


150,600 
506,200 
125,700 


117,100 

21,900 

NAp 


2.4 
2.7 
2.1 


3.6 
7.1 
NAp 


5,205 

19,681 

3,801 


6,070 


Intermediate. ...................... 


2,239 


Deep 


NAp 


Total 


782,500 


139,000 


NAp 


NAp 


28,687 


8,309 


Southern Florida: 

Shallow 


108,000 
548,900 
235,100 


32,700 
172,900 
656,900 


3.8 
4.2 
5.6 


3.8 
4.2 
5.6 


5,910 
33,197 
18,958 


1,789 


Intermediate 


10,457 




52,972 


Total 


892,000 


862,900 


NAp 


NAp 


58,065 


65,218 


Southeastern Coastal Plain 


2,751,400 


2,431,500 


NAp 


NAp 


250,355 


135,303 



NAp Not applicable. 
'Northeast Florida is combined with 
deposition. 



Southeast Georgia Embayment to constitute 1 major area of 



TABLE B-2. - Extent of environmentally sens 


itive areas for depos 


it zones. 


hectares 






Total 
area 


Environmentally sensitive 


areas 


Resource 


Region and depositional environment 


Coastal ' 


Water^ 


Managed 
land^ 


Urban" 


area 


Eastern North Carolina: 

Shallow 


118,500 
360,400 
332,300 


(5) 
(5) 


19,500 

100,200 

63,700 






2,800 


1,300 
3,300 
1,900 


97,700 


Intermediate ........................... 


256.900 


Deep 


263,900 


Total 


811,200 


(5) 


183,400 


2,800 


6,500 


618,500 


Southeast Georgia Embayment:^ 

Shallow 

Intermediate 

Deep > 


274,000 

1,558,700 

819,200 


19,900 

89,200 

161,700 


17,800 

109,800 

74,800 




148,300 

15,100 




122,700 

4,200 


236,300 

1,088,700 

563,400 


Total 


2,651,900 


270,800 


202,400 


163,400 


126,900 


1,888,400 


Northeast Florida:^ 

Shallow 


356,400 
648,900 
207,100 


800 
31,700 
52,700 


42,000 
63,100 
12,000 


45,200 

16,200 




700 

9,800 

16,700 


267,700 


Intermediate. 


528,100 


Deep 


125,700 


Total 


1,212,400 


85,200 


117,100 


61,400 


27,200 


921,500 


Southern Florida: 

Shallow 


194,400 

994,800 

1,279,000 


(5) 
(5) 
(5) 


40,600 
237,100 
325,900 




8,600 




13,100 
27,300 
61,100 


140,700 


Intermediate 


721,800 


Deep 


892,000 


Total 


2,468,200 


(5) 


603,600 


8,600 


101,500 


1,754,500 


Southeastern Coastal Plain 


7,143,700 


356,000 


1,106,500 


236,200 


262,100 


5,182,900 



Includes barrier islands and tidal marshes. 

Includes flood plains, lakes, rivers, and inland marshes. 
^Includes wildlife refuges and State and Federal forests. 

Incorporated limits of major cities; military bases. ^Coastal areas excluded from initial area. 

Northeast Florida is combined with Southeast Georgia Embayment to comprise 1 major area of deposi- 
tion; data courtesy of and modified from S. R. Riggs, Department of Geology, East Carolina University, 
Greenville, NC. 



A-U.S. GPO: 1985-505-019/20,098 



INT.-BU.OF MINES,PGH.,PA. 28088 



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